1. Field of the Invention
The present invention relates to a top-emitting N-based light emitting device and a method of manufacturing the same, and more particularly, to a top-emitting N-based light emitting device, which has improved ohmic characteristic and luminous efficiency, and a method of manufacturing the same.
2. Description of the Related Art
Nowadays, a transparent conductive thin film layer is being widely used in the fields of displays and energy industries.
In the field of light emitting devices, much research into transparent conductive thin-film electrodes that enable smooth hole injection and highly efficient light emission has been conducted.
Transparent conductive oxides (TCOs) and transparent conductive nitride (TCN) are materials for forming transparent conductive thin film layers and have lately been laboriously studied.
TCOs are, for example, indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), and indium tin oxide (ITO). TCN is, for example, titanium nitride (TiN).
However, these TCOs or TCN have a high sheet resistance, a high reflectivity, and a small work function. Thus, it is difficult to apply only one of the TCOs and TCN to a p-type transparent electrode of a top-emitting GaN-based light emitting device.
More specifically, first, when a thin film is formed of one of the above-described TCOs or TCN using physical vapor deposition (PVD), an e-beam or heat evaporator, or sputtering, the thin film has a high sheet resistance of about 100 Ω/cm2. Such a high sheet resistance of the thin film is an obstacle to the horizontal (parallel to an interfacial surface between films) current spreading of a light emitting device and the efficient vertical hole injection. Accordingly, it is difficult to apply one of the above-described transparent conductive materials to highly luminous light emitting devices, which have large area and high capacity.
Second, the above-described transparent conductive materials have a high reflectivity with respect to light emitted from GaN-based light emitting diodes, thus degrading luminous efficiency.
Third, since transparent conductive materials including ITO and TiN have a relatively small work function, it is difficult to form an ohmic contact between the transparent conductive materials and p-type GaN.
Finally, if an electrode that is directly ohmic-contacted with GaN-based compound semiconductor is formed using TCO, during formation of a thin film for the electrode, Ga2O3, which is an insulating material, is generated on the surface of GaN due to a high oxidation activity of Ga. This precludes formation of a good ohmic-contact electrode.
Meanwhile, when a light emitting device, such as a light emitting diode (LED) or a laser diode (LD), is formed using GaN-based compound semiconductor, a structure in which an ohmic contact between semiconductor and an electrode is formed is necessarily required.
GaN-based light emitting devices can be categorized into top-emitting light emitting diodes (TLEDs) and flip-chip light emitting diodes (FCLEDs).
In the prevalent TLEDs, light is emitted through an ohmic contact layer that contacts a p-type clad layer. Also, in order to embody a highly luminous TLED, a current spreading layer as an excellent ohmic contact layer is essentially required to compensate for a high sheet resistance of a p-type clad layer having a low hole concentration. Since the current spreading layer has a low sheet resistance and a high transmittance, smooth hole injection and current spreading and efficient light emission are enabled.
A conventional TLED includes a Ni layer and an Au layer, which are sequentially stacked on a p-type clad layer.
As is known, if the Ni/Au layer is annealed in an O2 atmosphere, a semi-transparent ohmic contact layer, which has an excellent specific contact resistance of about 10−3 to 10−4 Ω/cm2, is formed. When the semi-transparent ohmic contact layer is annealed in an O2 atmosphere at a temperature of about 500 to 600 □, the low specific contact resistance of the semi-transparent ohmic contact layer reduces Schottky barrier height (SBH) and thus, facilitates supply of carriers, i.e., holes, to the vicinity of the surface of the p-type clad layer.
Also, if the Ni/Au layer formed on the p-type clad layer is annealed, reactivation occurs. In other words, the dopant concentration of Mg in the surface of GaN is increased by removing Mg—H intermetal compounds, so that the effective carrier concentration in the surface of the p-type clad layer exceeds 1018/cm2. As a result, a tunneling phenomenon occurs between the p-type clad layer and the ohmic contact layer containing nickel oxide and thus, the ohmic contact layer has a low specific contact resistance.
However, a TLED having a semi-transparent thin-film electrode formed of Ni/Au contains Au, which degrades transmittance. As a result, the TLED has a low luminous efficiency and thus, cannot be one of the next-generation light emitting devices that have a high capacity and a high luminance.
Also, an FCLED includes a reflective film and radiates light through a transparent substrate formed of sapphire in order to increase heat emission and luminous efficiency during the driving of the FCLED. However, the FCLED has a high resistance due to oxidation and poor adhesion of the reflective film.
To overcome the drawbacks of the TLEDs and FCLEDs, a structure in which a TCO excluding Au, for example, ITO, instead of a conventional Ni/Au layer is used as a p-type ohmic contact layer so as to obtain a high transmittance was disclosed in many papers (e.g., IEEE PTL, Y. C. Lin, et al. Vol. 14, 1668 and IEEE PTL, Shyi-Ming Pan, et al. Vol. 15, 646). In recent years, a paper (Semicond. Sci. Technol., C S Chang, etc. 18(2003), L21) has presented a TLED that employs an ITO ohmic contact layer and has better output power than a TLED having a conventional Ni/Au film. However, although these ohmic contact layers can increase the output power of light emitting devices, they require relatively high operating voltages. Accordingly, since many problems still remain unsolved, the foregoing ohmic contact layers cannot be readily applied to light emitting devices that have a large area, a high capacity, and a high luminance.
In the meantime, LumiLeds Lighting (US) has reported that an LED having a high transmittance and excellent electrical characteristics was manufactured by combining oxidized thin Ni/Au (or Ni/Ag) and ITO (U.S. Pat. No. 6,287,947 by Michael J. Ludowise et al.). However, an ohmic-contact electrode forming process is very complicated, and because an ohmic electrode formed of transitional metals including Ni or oxides of elements of the group II of the Mendeleev Periodic Table has a high sheet resistance, it is difficult to embody a highly efficient light emitting device. Further, it is known that oxides of metals including Ni hardly have a high transmittance.
In summary, it is very difficult to obtain a transparent electrode including a good ohmic contact layer because of the following problems.
First, p-type GaN has a low hole concentration and thus, has a high sheet resistance of 104 Ω/cm2 or higher.
Second, since there is no material for the transparent electrode having a larger work function than the p-type GaN, high Schottky barrier is formed between the p-type GaN and the transparent electrode, thereby precluding efficient vertical hole injection.
Third, the electrical characteristics of most materials are inversely proportional to the optical characteristics thereof. Thus, a transparent electrode having a high transmittance generally has a high sheet resistance so that horizontal current spreading is remarkably degraded.
Fourth, while a TCO is being directly deposited on the p-type GaN, Ga2O3, which is an insulating material, is produced on the surface of the GaN, thus deteriorating the electrical characteristics of a light emitting device.
Finally, more heat is generated between the p-type GaN and the transparent electrode due to the above-described problems, so that the life span of the light emitting device is shortened and the operating reliability thereof is degraded.